Automotive vehicles have used hydraulic systems for many years to provide power assistance to the vehicle steering systems. The hydraulic systems typically consist of an engine driven pump that provides hydraulic fluid under pressure to an actuator which is connected into the steering system. A torsion bar senses the rotational torque and direction that the driver applies to the vehicle steering wheel, and appropriate hydraulic valving applies the hydraulic fluid in a manner that applies assisting forces to the steered wheels of the vehicle.
Over the years many improvements have been implemented to increase the efficiency and reduce the power consumption of the hydraulic systems. Despite these, the typical power steering system requires substantial amounts of energy because of inherent factors such as pumping losses.
Electrically assisted power steering systems offer increased efficiencies and packaging conveniences, and a great deal of development effort is being applied to prepare electrical power steering systems, also known as steer-by-wire systems, for the marketplace. Proposals for electrically powered vehicles add to this interest as the energy requirements of hydraulic power steering systems would likely prevent their use in electrically powered vehicles.
In a typical electrically assisted power steering system, angular displacement of the steering wheel is detected and converted into an electrical signal. The electrical signal is processed and applied to a servo motor that is attached to the steerable wheels of the vehicle. Much of the art of electrically assisted power steering systems is directed at devices for detecting the position and direction of rotation of the steering wheel, and at systems for processing and shaping the signals that are applied to the servo motor in efforts at duplicating the characteristics of the conventional steering system. U.S. Pat. No. 5,347,458 shows a steerby-wire system of interest in this regard.
Some work has been directed at providing electrical or electronic signals representative of the position of the steerable wheels of the vehicle. U.S. Pat. No. 5,275,250 discusses the use of potentiometers as position sensing devices for a vehicle steering control device. In U.S. Pat. No. 5,314,036, an elongated strip on the rack bar of a steering gear is designed with a signal pattern of magnetic characteristics. The rack bar is the output member of the steering gear and it is mechanically representative of the position of the steerable wheels of the vehicle. A sensor is located adjacent the strip and reads the magnetic characteristics in a manner that identifies the location of the rack bar and hence the angular location of the steerable wheels.